Information Technology Reference
In-Depth Information
Table 2-2
Comparison of iBGP and eBGP
iBGP
eBGP
Peering inside an AS must consist of a full mesh,
because an iBGP speaker is not allowed to pass on
prefixes learned from one iBGP peer to another
iBGP peer.
Has no full-mesh requirement.
Advertises the LOCAL_PREF attribute, which
eBGP does not. Does not modify the next hop and
AS_PATH (exceptions are discussed in Chapter 7).
Next-hop reachability within the AS is provided
by an IGP.
Modifies next-hop and AS_PATH attributes.
Does not require direct connectivity, because
iBGP uses an IGP to allow reachability to a remote
next hop.
Requires direct connectivity by default. In
almost all cases, a common IGP is not shared
between eBGP peers. Cisco IOS software
provides a workaround to this requirement
with the use of eBGP multihop.
Requires prefix synchronization between iBGP
and IGP to prevent routing loops and black-holing
of traffic.
Prefix synchronization
means that a prefix
learned via iBGP is not included in the best-path
selection unless that same prefix exists in the IGP.
If the IGP is OSPF, the router ID for the prefix in
the IGP must also match the router ID of the BGP
peer advertising the prefix.
Synchronization may be disabled if this AS does
not provide transit for another AS or if all routers
in the transit path are running BGP. It is generally
a good practice to disable prefix synchronization.
Has no synchronization requirement.
Routes from iBGP are not redistributed into an
IGP by default, even if the redistribution from
BGP into the IGP is configured, because this could
result in routing loops. If it is mandatory that iBGP
prefixes be redistributed into an IGP, IOS provides
the
bgp redistribute-internal
command.
Has no such restriction.
Path Decision Process
BGP steps through a complex algorithm to determine the best path and updates the BGP
RIB and IP RIB. As mentioned earlier, BGP is a policy tool. The significance of this is best
shown by how BGP uses attributes and other parameters to select the best path.
When multiple valid BGP paths to a particular destination exist, IOS lists them in the
reverse order in which they were received. That is, the newest path is listed at the beginning,
