Posted on Sunday, 16th April 2006 by sean
Multilink PPP is an add on to PPP encapsulation that lets you bond multiple links into one. It can be used over asynchronous dialup to bond multiple modem or ISDN connections, or to bond multiple synchronous serial lines such as T1s.
Recall that the common encapsulation types over a synchronous serial line are frame-relay, HDLC and PPP. The latter two are the ones normally used over leased lines.
With two links in parallel, the router will perform equal cost load balancing depending on how the routing protocol is configured via the maximum-paths command. By default there are 4 paths for IGPs.
Equal cost load balancing is nice, but with CEF on you get per destination load balancing, meaning a single flow can only consume one link. Those that work with VoIP will also know about serialization delay, which is caused by one packet waiting for another packet to be transmitted over the wire. PPP lets us fragment packets before they go out so that no single packet has to wait as long.
Configuration of MPPP is straightforward. With two routers configured back to back (S0-S0, S1-S1), the configuration of separate links looks like:
interface Serial0
ip address 2.2.2.1 255.255.255.252
encapsulation ppp
!
interface Serial1
ip address 2.2.2.5 255.255.255.252
encapsulation ppp
Moving to MPPP is like creating dial interfaces... Configure the physical interfaces, then a logical interface for the group, and relate the two at the physical level:
interface Multilink1
ip address 2.2.2.1 255.255.255.252
ppp multilink
multilink-group 1
!
interface Serial0
no ip address
encapsulation ppp
ppp multilink
multilink-group 1
!
interface Serial1
no ip address
encapsulation ppp
ppp multilink
multilink-group 1
! some versions of IOS use the form of ppp multilink group X instead
Now, the routing table shows the routes via the Multilink interface:
D 1.1.1.1 [90/3516928] via 2.2.2.2, 00:02:50, Multilink1
In general, with MPPP on, anything to do with IP goes at the Multilink interface level rather than the individual interface level.
Since PPP takes care of link testing, one can knock down links in the bundle without causing a flap in routing, and only minor loss.
Showing the status of the multilink bundle is easy enough, you can either look at the interface, or get details about MPPP with:
r1#show ppp multilink
Multilink1, bundle name is r0
Bundle up for 00:05:53
0 lost fragments, 0 reordered, 0 unassigned
0 discarded, 0 lost received, 1/255 load
0x5B received sequence, 0x5C sent sequence
Member links: 2 active, 0 inactive (max not set, min not set)
Se0, since 00:20:40, last rcvd seq 00005A
Se1, since 00:16:54, last rcvd seq 000059
To enable packet interleaving:
ppp multilink fragment-delay 20
ppp multilink interleave
The fragment delay calculates the maximum size of the packet allowable to obtain the desired serialization delay, and fragments smaller than that. In VoIP, packets generally contain 10-20 ms of voice data, which is where the above 20 msec comes from.
Configuring Link Fragmentation and Interleaving for Multilink PPP
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