Protocols and standards
• RFC4271: A Border Gateway Protocol 4 (BGP-4)
• RFC3392: Capabilities Advertisement with BGP-4
• RFC2918: Route Refresh Capability for BGP-4
• RFC1997: BGP Communities Attribute
• RFC2796: BGP Route Reflection
• RFC4724: Graceful Restart Mechanism for BGP
BGP extensions
Route reflection
By design, IBGP peers do not advertise iBGP routes to other iBGP peers. In order for iBGP peers to learn all the
routes within the autonomous system as well as all the external routes, the iBGP peers would have to be fully
meshed. This means for
n
iBGP peers there would have to be
n*(n-1)/2
iBGP sessions. In a large autonomous
system network configuration would become an issue.
Route Reflection is one of the alternate solutions to alleviate this problem. In the BGP network, one of the iBGP
speakers is designated as the route reflector. The route reflector advertises the routes it learns to other iBGP
peers.
In a route reflector configuration the other iBGP peers are classified as clientpeers and non-client peers.
The action taken by the route reflector (after determining the best route) depends on whether the best route was
received from a client peer or a non-client peer. If the route was received from a client peer, the route reflector will
reflect that route to all the client peers and non-client peers.
If the route was received from a non-client peer, then the route is advertised to all its configured clients.
Route reflection introduces two new discretionary attributes: Originator ID and Cluster List, which are used in
determining the best path as defined in
on page 375.
In an Autonomous System more than one route reflector can be configured.
BGP graceful restart (GR)
When a BGP speaker shuts down, planned or unplanned, the routes that are advertised by the speaker and
reachable via the speaker now become unreachable. Upon detecting that the BGP speaker has restarted, the
peers delete the routes and re-add them when the restarting router advertises them again. This results in route-
flap across the BGP connectivity and impacts multiple routing domains causing transient instability in the network.
The Graceful Restart capability is supported as a ‘helper router’ on the switches. In 'helper only' mode the router
helps the other restarting router by holding the received routes from it as stale routes and not dropping them.
1.
To establish a BGP session with a peer, a BGP GR Restarter sends an OPEN message with GR capability to
the peer.
2.
Upon receipt of this message, the peer is aware that the sending router is capable of Graceful Restart, and
sends an OPEN message with GR Capability to the GR Restarter to establish a GR session. If neither party
has the GR capability, the session established between them will not be GR capable.
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Aruba 3810 / 5400R Multicast and Routing Guide for ArubaOS-
Switch 16.08
