Low Memory Handling

Memory problems on routers is nothing new. It is generally less of a problem in current day, but is still seen from time to time.

BGP is capable of handling large amount of routes and in comparison to other routing protocols, BGP can be a big memory hog. BGP peering devices, especially full internet peering devices, require larger amounts of memory to store all the BGP routes. Thus it’s not uncommon to see a BGP router run out of memory when a certain route count limit is exceeded.

A router running out of memory, commonly called Low Memory, is always a bad thing. The result of low memory problems may vary from the router crashing, to routing processes being shut down or if you lucky enough erratic behavior causing route flaps and instability in your network. None which is desired.

Low memory can be caused by any of the following:

  •     Partial physical memory failure.
  •     Software memory bugs.
  •     Applications not releasing used memory chunks.
  •     Incorrect configuration.
  •     Insufficient memory allocation to a Nexus VDC.

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Output 101 : BGP AFI/SAFI

When BGP peers set up their session between them, they send an OPEN message possibly containing optional parameters.

One optional parameter is capabilities. Possible capabilities are Multiprotocol extensions, route refresh, outbound route filtering (ORF), and so on. When the BGP peers exchange the Multiprotocol extension capability, they exchange AFI and SAFI numbers and thus identify what the other BGP speaker is capable of.

IPv6 in BGP is implementated via Multi-Protocol BGP (MPBGP) (RFC 2283), as is MPLS and VPN’s through two new attributes: MP_UNREACH_NLRI and MP_REACH_NLRI. The first two values in these two attributes contain the Address Family Identifier (AFI) and the Subsequent Address Family Identifier (SAFI).

AFI Meaning
1 IPv4
2 IPv6
SAFI Meaning
1 Unicast
2 Multicast
3 Unicast and multicast
4 MPLS Label
128 MPLS-labeled VPN

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R&S Quick Notes – BGP


  • When using Communities, don’t forget “neighbor send-community”
  • Know your attributes and the direction which applied, when to used what.
  • “aggregate address” needs a more specific prefix in the BGP table for aggregate to be advertised.
  • Synchronization issue has 3 solutions, 1- Load BGP on all transit routers, 2- GRE tunnel, 3- Redistribution BGP>IGP.
  • “no bgp nexthop trigger” – Disables next-hop tracking between scanner intervals.
  • “no bgp fast-ext-fallover” – Force the router to wait for the dead-timer to expire, before generating notification messages , when a connected peer goes down.
  • “neighbor fall-over” – Will check neighbor connenctivity between scanner intervals, aka BGP Fast Peering.
  • Only the Holdtime is sent in update-msg. Two neighbors will use the lowest holdtime and then calculate the keepalive from that.
  • Know your Regular Expressions
  • Know the difference between Peer-Groups and Peer-Templates

BGP Conditional Route Advertisement

Assume the following scenario:


BOB’s network resides in AS-300.

For what ever reason, AS-300 needs all traffic to Prefix-A ( to come via ASN-200 only, but in the event of link failure between AS-200 and AS-300 traffic are allowed to come in from AS-100. With the normal BGP attributes you can do this really quickly.

Hypothetically,  lets assume for a second that AS-100 has a weight set, preferring the direct link to AS-300 for all prefixes learned from AS-300.  And AS-100 is not cooperating with your request, nor is  interested to make any changes in their AS. How can you influence AS-100’s  decision, to prefer the route learned via AS-200.

There is a nice and not-so-nice to way to do this.

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Clearing BGP Sessions

There are a couple ways, some more recommended than others, to reset or clear a BGP session between two neighbors.

The Cisco IOS Software Command Summary lists the following circumstances under which you must reset a BGP connection, for changes to take effect:

  • Additions or changes to BGP-related access lists
  • Changes to BGP-related weights
  • Changes to BGP-related distribution lists
  • Changes to the BGP-related timer’s specifications
  • Changes to the BGP administrative distance
  • Changes to BGP-related route maps


Traditional clearing of BGP session (aka Hard Reset)

  • Tears down the BGP session  with all Neighbors, a specific neighbor or peer group.
  • A new session is re-established within 30-60 seconds, depending.
  • Processing of the full Internet table can take a really long time.
  • This is almost NEVER recommended on production networks, due the disruptive behaviour.

router#clear ip bgp {* | neighbor ip | peer-group}

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