Timers, Elections & Defaults

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It is often difficult to remember various aspects of all the different protocols out there in network engineering – timer values, behavioural defaults and election winning criteria are some of the hardest things to keep straight in your head. This page is intended to be a quick reference for just such hard to remember facts.

It will be updated overtime and, I imagine, always be woefully incomplete given the number of protocols in existence. However my hope is that this can be a page you can have open on the side while studying to refresh yourself on the gritty specifics that just slip your mind.

If Vendor/Platform field just mentions Cisco (rather than IOS or XR specifically) you can assume it applied to both IOS and XR. Juniper refers to SRX.

BGP

Feature Vendor/Platform Default Value/Metric Comment
Communities Cisco XR By default, XRs advertises both standard and extended to iBGP peers. They do not advertise then to eBGP peers. Use commands send-community-ebgp and send-extended-community-ebgp under the neighbor address family to send standard and extended respectively.
Communities Cisco IOS and Cisco NXOS Neither standard nor extended are advertised to either iBGP or eBGP. Enable per neighbor using neighbor x send-communty [standard|extended|both]
Administrative Distance for iBGP Cisco 200 n/a
Administrative Distance for iBGP Juniper  170 n/a
Administrative Distance for eBGP Cisco 20 n/a
Administrative Distance for eBGP Juniper 170 n/a
Hello Timer Cisco 60 n/a
Weight Attribute Cisco 32,768 if locally originated. 0 otherwise. Higher is better This is Cisco Proprietary.
Local Preference Cisco Default 100. Higher is better. Well-Known Discretionary attribute.
Domain ID when redistributing from OSPF to BGP Cisco IOS Automatically encodes it based on process ID. n/a
Domain ID when redistributing from OSPF to BGP Cisco XR Does not encode it. This adheres to RFC 4577
Redistributing different OSPF route types into BGP from OSPF Cisco Only intra and inter areas routes are redistributed. External routes are not. External route can be redistributed by adding the external 1 external 2  command to the end of the redistribute command.
MED Cisco Default is 0, unless redistributing from OSPF in which case it is the OSPF cost. Lower is better n/a
MED Comparison Cisco Only compared if the neighboring AS is the same for competing routes. Routes are compared from newest to oldest Use IOS command bgp always-compare-med and XR command bgp  bestpath med always to force MED to be compared regardless of neighboring AS. The deterministic med feature will group paths from common ASes together during BGP prefix comparison rather than going from oldest to newest.
Address-family ipv4 unicast Cisco IOS This is enabled by default whenever configuring a neighbor. Regardless of whether or not you use the ipv4 address family. Disable this behaviour using no bgp default ipv4-unicast BGP command under the global process.
Router ID Cisco IOS In order of preference:

  1. Configured Router ID (using bgp router-id <id|int> command.
  2. Highest numbered loopback.
  3. Highest physical interface that is up.
Changing the BGP Router ID will cause sessions to flap.
Router ID Cisco IOS-XR In order of preference:

  1. Configured Router ID (using bgp router-id command.
  2. Highest numbered loopback.
Changing the BGP Router ID will cause sessions to flap. Not that IOS-XR will not select a physical interface IP as its Router ID.
Inbound update filtering order of execution Cisco IOS
  1. route-map
  2. filter-list (AS_PATH access list)
  3. prefix -list
  4. distribute-list
n/a
Outbound update filtering order of execution Cisco IOS
  1. filter-list
  2. route-map | unsupress-map
  3. advertise-map
  4. prefix-list
  5. distribute-list
  6. ORF prefix-list
n/a
AS notation Cisco asplain notation asplain notation makes the AS number appear as a single integer. Change to as-dot notation using bgp asnotation dot IOS command under BGP or as-format asdot global XR command.
TCP MSS size for BGP session between two peers Cisco IOS-XR 1240 bytes If MTU path discovery is enabled this will change to the maximal values of 1460
BGP Keepalive Cisco 60 seconds Change using the BGP subcommand timers bgp . The command is the same for IOS and XR
BGP Holdtime Cisco 180 seconds Change using the BGP subcommand timers bgp . The command is the same for IOS and XR

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OSPF

Feature Vendor/Platform Default Value/Metric Comment
Redistribution metric from non-OSPF protocol. Cisco Metric defaults to 20 when redistributing into OSPF. Unless source is BGP, in which case the metric will be 1. This is assuming no MED is set in the BGP being distributed. Also, the Default OSPF network is E2.
Reflood Timer Cisco 30 minutes A router will reflood LSAs every 30 minutes after incrementing the sequence number by 1
maximum-paths Cisco 4 The number of equal cost OSPF paths that can be added to the routing table. OSPF doesn’t support unequal cost load balancing.
Reference Bandwidth Cisco 100Mbps Used in metric calculation (metric = Reference_Bandwidth/ Interface_Bandwidth) . Change the reference bandwidth using auto-cost reference-bandwidth (IOS)
Summary Route Metric Cisco Lowest metric of all routes that it is advertising This pertains to inter-area summarisation using the area range [cost ] command.
default-information originate metric

and

default metric for stub area default routes.

Cisco 1 If not specified, the default information originate  IOS command (used to send a type 5 LSA into an OSPF area as a default) applies an OSPF cost of 1 to the default route. The default metric-type used is 2.
Network Type on WAN Link (like serial) or Tunnel Cisco Point-to-point n/a
Network Type on LAN/Ethernet Cisco Broadcast n/a
Hello on point-to-point and broadcast networks Cisco 10s n/a
Dead Timer on point-to-point and broadcast networks Cisco 40s n/a
Hello on non-broadcast and point-to-multipoint (broadcast or not) networks Cisco 30s n/a
Dead Timer on non-broadcast and point-to-multipoint (broadcast or not) networks Cisco 120s n/a
DR Election winner Cisco In order of preference:

1. Highest interface priority wins (default 1, max 255)

2. Highest Router ID

On a DR is elected the process is repeated for the BDR
Flooding of TE information Cisco Done every 3 minutes Configuration, Link and Bandwidth changes will trigger these updates to take place sooner.

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IS-IS

Feature Vendor/Platform Default Value/Metric Comment
Metric Cisco 10 n/a
Network Type Cisco Broadcast n/a
Level Cisco L1L2 n/a
Topology Mode Cisco IOS-XE Single-topology A mismatch will not prevent a neighborship forming but will prevent hinder the advertisement of v6 prefixes
Topology Mode Cisco IOS-XR Multi-topology A mismatch will not prevent a neighborship forming but will prevent hinder the advertisement of v6 prefixes
Metric Style Cisco Narrow This only allows a maximum metric of 63 if not changed to wide. Multi-topology requires metric style wide.
Hello Timer Cisco 10s Change with IOS command isis hello-interval [level]
Holdtime Multiplier Cisco 3 This means a neighbor is seen as dead after 3 hello intervals have passed. Change with IOS command isis hello-multiplier [level]
LSP Lifetime Cisco 20mins If not refreshed, and if this counter reaches zero the LSP will be purged (meaning the LSP header is advertised with an empty body and lifetime set to 0).
LSP Refresh Cisco 15mins This resets the LSP lifetime. Changing using Cisco IOS cli command lsp-refresh interval
ZeroAgeLifeTime Cisco 60s If LSP has been purged (after 20 mins with no refresh) the expired LSP will be deleted from the database after this timer has expired.
Adjacency Handshake Cisco Cisco 3-way handshake The newer 3 way handshake is using, utilising IIHs with TLVs including neighbor system and extended circuit IDs. Set this manually using isis three-way-handshake cisco interface command
Flooding of TE information Cisco Done every 3 minutes Configuration, Link and Bandwidth changes will trigger these updates to take place sooner.

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PIM Sparse Mode

Feature Vendor/Platform Default Value/Metric Comment
Bootstrap BSR Priority Cisco Range 0 – 255. Default 0. Highest wins. n/a
Designated Forwarder election on Multi-access networks Cisco Higher IP wins if routing protocol AD and metric are a tie. n/a
Bootstrap start Timer Cisco 130s. Started once Bootstrap is configured. n/a
Bootstrap timer Cisco If router deems itself to be the BSR it sents a Bootstrap message every 60 seconds. n/a
Bootstrap Candidate-RP Priority Cisco Range 0-255 n/a
Auto-RP RP election Cisco Highest IP wins n/a
Join/Prune periodic keepalives Cisco 60 seconds. Also applies to dense mode
Join/Prune holdtime  Cisco Keepalive interval x 3.5 (so 210s by default) Also applies to dense mode
Designated Router value Cisco 1 Change using ip pim dr-priority interface command
Designated Router Election Cisco Highest DR Priority. Highest IP used in case of tie-breaker n/a
PIM Assert Cisco 1. Lowest AD for route to source.
2. Best metric to the source.
3. Highest router IP.
PIM asset concerns forwarding on multicast traffic over shared medium. It shouldn’t be confused with DR or IGMP Querier.

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IGMPv2

Feature Vendor/Platform Default Value/Metric Comment
General Query Interval Cisco Sent by the IGMP querier every 60 seconds Send to 224.0.0.1
Group-Query Cisco Send whenever a leave message is received. Sent twice, 1 second apart. Destination address is multicast group for which the leave was received
Group Membership Interval Cisco 3 x the General Query Interval Think of this as the Dead Timer for multicast group. Routers will remove them from the IGMP tables if no reports are received in response to queries  within this time.
Querier Cisco The router with the lower IP with be the querier if multiple IGMP enabled routers are on a LAN segment The querier is responsible for sending General and Group-Specific queries
Other Querier Present Interval Cisco 2 x General Query Interval If the current querier is not hear from within this time another router will take over

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Quality of Service

Feature Vendor/Platform Default Value/Metric Comment
CB-WFQ, max-reserved-bandwith percentage Cisco 75% Use max-reserved-bandwidth to change
LLQ, Priority queue policer burst rate Cisco 20 percent This is the default if not specified
WRED, Exponential Weighting constant Cisco 9 Results in 0.2% of the new average being account for at each average queue depth calculation.
Shaping, Tc Cisco IOS 125ms Not the default for all shaping tools, but the default for many.
Shaping, Class Based shaping default queuing method Cisco IOS FIFO WFQ, LLQ and CB-WFQ are support but FIFO is the default.
Shaping, Be (Burst Excess) Cisco IOS Bc value If not specified the Be value will be equal to the Bc value
Shaping, Bc (Burst commit) when Shaping speed is less than or equal to 320 kbps Cisco IOS 8000bits n/a
Shaping, Bc (Burst commit) when Shaping speed is over 320 kbps Cisco IOS Tc is set to 25ms and calculated Bc using bc = Tc * CIR n/a
Policing, Bc Cisco IOS The larger (in bytes) of 1500 or CIR/32 CIR is measured in bps
Policing, Be for single rate, two colour policer Cisco IOS 0 bytes n/a
Policing, Be for single rate, three colour policer Cisco IOS Equal to the Bc value in bytes n/a
Policing, Be for dual rate, three colour policer Cisco IOS The larger (in bytes) of 1500 or PIR/32 PIR is measured in bps

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MPLS

Feature Vendor/Platform Default Value/Metric Comment
LDP – Router ID Cisco 1. Configured address using mpls ldp router-id (in IOS)

2. Highest numbered loopback

3. Highest numbered interface IP.

LDP Router ID (which is in turn the highest loopback ID)

n/a
LDP – Transport Address Cisco This is the LDP router ID and is included in LDP Hello messages. The transport address is used to establish the TCP sessions between LDP neighbors. This can be changed in IOS using mpls ldp discovery transport-address [interface | ip-address]
LDP Discovery Hello Cisco 5s Change using mpls ldp discovery hello interval IOS command
LDP Discovery Hold Time Cisco 15s Change using mpls ldp discovery holdtime interval IOS command
MPLS VPN – Service Label Allocation Cisco Uses per-prefix mode Different label sets are kept for ipv4 and ipv6 (even if using per-vrf)
MPLS VPN – Service Label Allocation Junos Users per-vrf The same label will be allocated for both v4 and v6 labels with the same VRF
LDP Session Holdtime Cisco 180 seconds This is the holdtime for the TCP session between neighbors and not the hello adjacency
LDP Session Keepalive interval Cisco 60 seconds n/a
LDP Session Protection Duration Cisco infinite Change on IOS using mpls ldp session protection duration command
MPLS TTL Propagation Cisco Enabled Disable using  no mpls ip propagate-ttl [forwarded|local] (IOS) or  mpls ip-ttl-propagate disable [forwarded|local] (XR)
LDP Session Protection Holdtime Cisco 864000 seconds n/a
LDP Control Mode Cisco Independent The LSR creates a local label for a FEC independent of other LSRs
LDP Control Mode Juniper Ordered The LSR will only create a local binding if it either recognises itself as the egress LSR for a given FEC or if it receives a label binding from the next-hop for the given FEC

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Segment Routing

 

Feature Vendor/Platform Default Value/Metric Comment
PHP-off flag in SID advertisements Cisco Unset This is a sort of double negative. If the PHP-off flag is unset, then the upstream neighbor will perform PHP.
Explicit-Null Flag in SID advertisements Cisco Unset If the Explicit-Null Flag is set, the PHP-off Flag must be set as well.
SRGB Label Range Cisco 16,000 – 23,999 Change using IOS-XR global command segment-routing global-block
SRLB Label Range Cisco 15,000 – 15,999 Change using IOS-XR global command segment-routing local-block
Router ID used in IS-IS SR Capability TLV Cisco IOS-XR In order of preference:

1, The configured MPLS-TE router-id.

2. The configured IS-IS router-id.

3. Lowest IPv4 loopback under IS-IS instance.

4, Lowest IPv4 non-loopback under IS-IS instance.

It is recommended this be set to one the /32 loopback of the device and also to make this same /32 the node-SID for the router.
SR-TE Candidate Path Preference Cisco The default preference is 100 n/a
SR-TE Candidate Path Selection Cisco The valid candidate path with the highest preference  becomes the “Active” Path of the corresponding SR Policy All candidate paths must be valid
Weight assigned to SID List that is part of an Explicit Candidate Path Cisco 1 A SID List is invalid of the Weight is 0. The Weight value for a SID Lists controls the amount of traffic steered over the List, relative to other SID lists in the set.
Selecting an Adj-SID based on a Segment Descriptor that is an IP address on a point-to-point link Cisco The head-end router, if given the option of two Adj-SIDs (one protected and one unprotected) will select the protected Adj-SID by default when resolving the descriptor to its corresponding label n/a
Automated Steering Cisco Enabled by default If a headend H with a valid SR Policy P(C,E) receives a BGP route B/b with color C and next-hop E, then H automatically installs B/b via P in the forwarding plane.
Headend calculating candidate path, when local and PCE based options are available Cisco Headend will try to calculate path locally by default. Falling back to PCE if it must. If pcep is configured under the dynamic section on an ODR template (in XR), then only the SR PCE will be used.
Custom Flex-Algo range Cisco Identifier value will be between 128 and 255 Flex-Algo contains (1) A calculation Type, (2) Optimisation Objective (3) Constraints
Advertisement by IGP of local Flex-Algo definition Cisco Not done by default Use advertise-definition under flex-algo config mode under the IGP to advertise it out
Flex-Algo priority Cisco 128 n/a
Flex-Algo metric type Cisco IGP Use metric-type delay XR command to change to metric (under flex-algo config mode under the IGP)
Flex-Algo election Cisco 1. Highest Flex-Algo Priority

2. If IS-IS, highest system ID. If OSPF, highest Router ID

Both local and remote algorithm are considered. Local algorithms, however are only considered if they are advertised.

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Access Control

Feature Vendor/Platform Default Value/Metric Comment
RPL route policy fall-through action (no match) Cisco IOS-XR Deny pass and set in an RPL will keep process and accept if there are no other matches. drop will immediately reject the prefix.
Route-map fall-through action (no match) Cisco IOS Deny n/a
Route-map action if there is no match statement under entry Cisco IOS Accept everything n/a
ACL fall-through action (no match) Cisco Deny n/a

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IPv6

Feature Vendor/Platform Default Value/Metric Comment
Reachable Time Cisco Reachable Time is calculated using NUD Base Reachability Time x Random_number_between_0.5_and_1.5 This is included in RS messages
NUD Base Reachability Time Cisco 30 seconds Set using ipv6 nd reachable-time

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TCP

Feature Vendor/Platform Default Value/Metric Comment
Default TCP MSS value n/a 536 bytes This default will typically be set if the MSS is not included in the TCP SYN and SYN/ACK during the 3 way handshake.

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MPLS Traffic Engineering

Feature Vendor/Platform Default Value/Metric Comment
Maximum Reservable Bandwidth (as a link attribute) Cisco IOS 75% of total link bandwidth n/a
Maximum Reservable Bandwidth (as a link attribute) Cisco IOS-XR 0% of total link bandwidth n/a
Flooding of TE information (for both IS-IS and OSPF Cisco Done every 3 minutes Configuration, Link and bandwidth changes will trigger these updates to take place sooner.
Tunnel Setup Priority compared to Hold Priority Cisco The lower the Priority the more important. A tunnel with a lower Setup Priority than the Hold Priority of an existing tunnel can pre-empt the existing Tunnel. The Setup Priority should never be lower than the Hold Priority Cisco CLI will not allow you to configure a Setup Priority that is lower than a Hold Priority

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